1. Field of the Invention
The present invention relates to a flat backlight device used to illuminate a liquid crystal display panel from the rear surface side thereof, and a liquid crystal display device provided with the flat backlight device.
2. Description of the Related Art
Liquid crystal display devices are widely used. Each of the liquid crystal display devices includes a liquid crystal display panel and a flat backlight provided on the rear surface side thereof. In recent years, along with an increase in size of the liquid crystal display panel, the flat backlight becomes larger in size. The functions required for the large-size flat backlight are, for example, high and uniform intensity over the entire light emitting surface, being thin and lightweight, and a reduced manufacturing cost. In particular, along with increases in sizes of constituent members, the scale of an assembly line becomes larger to increase a capital investment. Under such circumstances, product failure is a main cause of an increase in manufacturing cost.
FIG. 6 is a schematic cross sectional view showing a conventional liquid crystal display device 100. A backlight 107 is provided under a liquid crystal display panel 105. The backlight 107 includes an outer case 106, two light sources 101a and 101b stored in the outer case 106, two-divided light guide members 104a and 104b, a reflecting plate 102 provided about the peripheries of the light sources 101a and 101b and under the light guide members 104a and 104b, and a diffusion plate 103 for diffusing light beams emitted from the light guide members 104a and 104b in a predetermined direction. In this example, in order to realize an increase in size of the backlight, the light sources 101a and 101b are disposed on the right and left sides and a light guide member is divided into two at the center thereof. A light beam is guided to the light source 101a located on the left side by the light guide member 104a located on the left side, and a light beam is guided to the light source 101b located on the right side by the light guide member 104b located on the right side (see, for example, JP 05-158035 A).
FIG. 7 is an exploded perspective view showing a conventional known surface light source device 110. In the surface light source device 110, divided optical films are sandwiched between two diffusion plates 112 and 115. That is, the optical films are a four-divided sub selective reflection type film 113 and a four-divided sub reflection type polarizing film 114. A plurality of light sources 111 stored in a frame portion 116 are arranged under the diffusion plate 112. In this example, each of the sub selective reflection type film 113 and the sub reflection type polarizing film 114 is divided to realize an increase in size of the surface light source device 110 (see, for example, JP 2006-120584 A).
In the backlight 107 shown in FIG. 6, however, the light sources 101a and 101b are provided in side regions of the outer case 106 to introduce light beams from end portions of the light guide members 104a and 104b. Therefore, there is a problem in that the amount of light becomes insufficient as the backlight 107 increases in size. When the light guide member 104 increases in size, the light guide member 104 can be physically divided into two or more. However, a light beam is reflected in a boundary between the divided light guide members. Therefore, there is a problem in that the light beam is unlikely to be emitted at a uniform intensity. In order to prevent the light beam from being reflected in the boundary, the respective light guide members can be bonded to each other. However, when a defect such as a scratch or stain is found, the bonded light guide members need to be replaced. Therefore, there is a problem in that the advantage of the divided light guide members cannot be taken.
In the surface light source device 110 shown in FIG. 7, the plurality of light sources 111 are attached to the frame portion 116 and provided under the diffusion plate 112. A cold cathode fluorescent lamp (hereinafter referred to as CCFL) is used as such a type of light source 111 which is a large surface light emitting source. In order to increase the intensity of light emitted from the diffusion plate 115 and obtain the uniform intensity over the entire surface thereof, a large number of CCFLs are employed. Therefore, there is a problem in that the frame portion 16 storing the CCFLs becomes thicker, which leads to an increase in weight. It is necessary to produce the CCFLs according to a light emitting surface size, so there is a problem in that the common use of constituent members is difficult to be achieved.
In recent years, an area control system capable of adjusting the intensity of a light emitting source for each area is employed for a large-size liquid crystal display device in some cases. According to the area control system, the amount of light from the backlight is adjusted for each area of the backlight according to a display screen of the liquid crystal display device. In the case of the liquid crystal display device 100 shown in FIG. 6, only the intensity on each of the right and left areas can be controlled. In the case of the surface light source device shown in FIG. 7, only the intensity on an area in a longitudinal or lateral direction can be controlled. Therefore, there is a problem in that a light controllable area is small. When the CCFLs are used as light sources, it is necessary to provide an inverter circuit for each of the CCFLs to adjust light, so there is a problem in that the number of constituent parts increases, which leads to increases in weight and volume.